Voltage switching apparatus for color kinescopes



Jan. 28 1969 W. H. CLINGMAN, JR

VOLTAGE swncumc APPARATUS FOR COLOR KINESCOPES Filed May '51. 1966 it? IFIGLI,

8 Claims ABSTRACT OF THE DISCLOSURE Disclosed is a voltage switchingapparatus which can be used for applying a high voltage square wave to acolor kinescope tube of the type in which phosphors of red and cyan arecoated on the inside face of the tube, the red phosphors being energizedby electrons of a first energy, and the cyan phosphors being energizedby electrons of a higher energy. The tube has a first transparentconducting layer which is coated upon its face over the phosphors, and asecond conducting layer coated on the neck portion of the tube therebypresenting a capacitive load to which mutually out-of-phase high voltagepulses formed by the switching apparatus of the invention can beapplied. The switching apparatus comprises two switching circuits, eachconnected to the primary winding of a high-voltage transformer, thefirst switching circuit being an SCR (silicon controlled rectifier)which is connected in parallel with a primary winding of the transformerand a capacitor and with its gate connected to a first trigger circuit,the second switching circuit being a second SCR connected from theprimary winding of a transformer to ground and with its gate connectedthrough a capacitor to a second trigger circuit, whereby when the SCRsare alternately triggered, a square wave is formed on the secondarywinding of the transformer. The square wave is then applied to theconducting layer on the face of the tube and the conducting layer in theneck of the tube to alternately accelerate the electrons emitted fromthe electron gun at high and low energies thereby to excite either thered or cyan phosphors.

This invention relates to voltage switching apparatus and moreparticularly to such apparatus for switching electron acceleratingvoltages in a color kinescope.

In color kinescopes of the variable penetration type employing aphosphor screen having a plurality of phosphors which emit light ofdifferent colors when struck by electrons of different energies, it istypically necessary to switch the screen voltage between at least twodifferent levels so that electrons directed toward the screen areaccelerated to at least two different energy levels for producing lightof different colors. The voltage changes required are typicallyrelatively high, being on the order of several kilovolts, and theswitching must be accomplished at a relatively rapid rate in synchronismwith the color signals applied to the kinescope, e.g., at a linesequential rate which requires the voltage to be switched more than10,000 times per second. Since the screen typically constitutes acapacitive load to the voltage source, it has been difficult to obtain asatisfactory stepped voltage waveform providing electron acceleratingvoltages at several distinct levels so as to produce satisfactory colorseparation.

Among the several objects of the present invention may be noted theprovision of voltage switching apparatus which will apply a steppedvoltage to a capacitive load such as the phosphor screen of the colorkinescope; the provision of such apparatus which will produce such astepped voltage which spans a voltage range of several nited StatesPatent Patented Jan. 28, 1969 kilovolts; the provision of such apparatuswhich will switch voltage levels at relatively high repetition rates;the provision of such apparatus providing a waveform which stepsrelatively abruptly from one distinct voltage level to another; theprovision of such apparatus which is relatively simple and inexpensive;and the provision of such apparatus which is highly reliable. Otherobjects and features will be in part apparent and in part pointed outhereinafter.

Briefly, voltage switching apparatus according to the present inventionis adapted for applying a stepped voltage to a capacitive load. Theapparatus includes a transformer having a primary winding and asecondary wi-nding for connection to the capacitive load. A firstswitching circuit including an SCR and a capacitor in series with eachother is provided for connecting the primary winding across a voltagesource. A second switching circuit including a second SCR is connectedacross the primary winding and the capacitor. When either of the SCRsare triggered, the leakage reactance of the transformer causes thecapacitive load to be charged to a respective voltage which then reversebiases that SCR thereby cutting off conduction therein. Thus, when theSCRs are alternately triggered, the capacitor is switched repetitivelybetween at least two different voltage levels each of which persistsuntil the next SCR is triggered.

The invention accordingly comprises the constructions hereinafterdescribed, the scope of the invention being indicated in the claims thatfollow the description.

In the accompanying drawings in which one of various possibleembodiments of the invention is illustrated,

FIGURE 1 is a partially schematic diagram of a color kinescope providedwith voltage switching apparatus of this invention for varying theelectron beam accelerating voltage;

FIGURE 2 is an equivalent circuit diagram of the voltage switchingapparatus of FIGURE 1;

FIGURE 3 is a graph representing the behavior of a voltage occurringwithin the voltage switching apparatus of FIGURE 1;

FIGURE 4 is a graph similarly representing a current flow; and

FIGURE 5 is a graph representing the same voltage as FIGURE 3 over arelatively long period of time.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

Referring now to FIGURE 1, there is indicated at 11 a color kinescope ofa type with which the present invention is useful. Kinescope 11 includesa conventional glass envelope 13 having a screen portion 15, a neckportion 17 and a bell-shaped intermediate portion 18 connecting the neckand screen portions. Coated on the inner surface of the screen portion15 is a phosphor screen or layer 19 which includes phosphors which emitlight of different colors when struck by electrons of differentenergies. Phosphor screen 19 may, for example, be constituted by amixture of two different kinds of phosphor particles one of which emitsred light when energized by electrons having energies above a relativelylow predetermined level and the other of which emits cyan light whenenergized by electrons having energies above a relatively highpredetermined level. Such a screen will emit red light when struck byelectrons at the relatively low level and white or substantiallyachromatic light when struck by electrons at the relatively high energylevel, which electrons can energize both of the phosphors. Suchred-white image displays are used in the art for the presentation ofcolor images and images so presented appear to have a relatively widerange of hues subjectively having a greater saturation than that whichis actually present in the colorimetric sense. Methods of preparingphosphors useful in 3 making such a screen are disclosed in copendingapplication Ser. No, 459,582, filed May 28, 1965.

Over phosphor screen 19 is deposited a film 21 of aluminum which isconductive and yet is also thin enough to be substantially electronpermeable. By means of film 21 suitable electron accelerating voltagesmay be applied to the phosphor screen 19. Aluminum film 21 also extendsbeyond the face portion 15 of kinescope 11 onto a preselected margin ofthe intermediate portion 18 of envelope 13 thereby constituting a firstconductive band 23 on the inner surface of the intermediate portion.

Within the neck portion 17 of envelope 13 there is mounted aconventional electron gun 27 for emitting a beam of electrons directedtoward phosphor screen 19. For the purpose of the example describedherein, it is assumed that this color display system is operated in aline-sequential mode. For this purpose a line-sequential, color videosignal is applied to gun 27 for varying the electron beam current, thatis, the rate at which electrons are emitted by the gun. The video signalthus controls the instantaneous brightness of the light produced by thebeam on phosphor screen 19. It should be understood, however, that othermodes of presentation, such as fieldsequential, may also be employed byappropriately varying the different switching rates describedhereinafter and applying a corresponding video signal to gun 27.

Electrons emitted from gun 27 pass through the magnetic influence of adeflection yoke 29. Yoke 29 is energized in conventional manner todeflect the beam of electrons in a scanning raster over the envelopeface portion 15. However, as is understood by those skilled in the art,the raster will be of uniform size only if the electrons emitted by gun27 are all accelerated to the same energy or if compensation is made forthe different deflection effects undergone by electrons having differentenergies.

The inner surface of the part of the intermediate envelope portion 18adjacent neck 17 is coated with a conductive band as indicated at 33thereby to constitute a generally annular, horn-shaped electrode whichis concentric with gun 27 and through which the beam of electronsemitted by the gun pass on their way to phosphor screen 19. Band 33 mayconveniently be constituted by a so-called dag coating on envelope 13.As is explained in greater detail hereinafter, electrode band 33 isemployed to exercise a radial corrective effect on the deflection of theelectron beam passing therethrough,

Electrical connections are made to the electrode band 33 and to thephosphor screen-covering aluminum film 21 as indicated at 35 and 37,respectively, and these connections extend through envelope 13 by meansof conventional feed-through terminals.

Screen 19 and electrode band 33 are provided with outof-phase steppedvoltages, and in particular with voltage waves of rectangular waveformand of several kilovolts amplitude, by the circuit indicated generallyat 41. For this purpose electrode band 33 and the screen band 23 areconnected to respective secondary windings W1 and W2 of a transformerT1. The opposite ends of the transformer secondary windings are providedwith respective D.C. biasing potentials. Appropriate nominal D.C.potentials for electrode band 33 and screen band 23 are approximately 12and 16 kilovolts, respectively. Transformer T1 also includes a primarywinding W3, one end of which is connected to ground through theanode-cathode circuit of an SCR (silicon controlled rectifier) Q1.Triggering signals applied to a terminal 43 are coupled to the gateterminal of SCR Q1 through a coupling capacitor C1. The other end ofwinding W3 is connected to a positive supply terminal 45 through a D.C.blocking capacitor C2 and a current limiting resistor R1. The voltageexisting between capacitor C2 and resistor R1 is smoothed by a filtercapacitor C3, Primary winding W3 and capacitor C2 together are shuntedby the anode-cathode circuit of second SCR Q2. Triggering signals forSCR Q2 applied 4 to a pair of terminals 47 and 49 are coupled to thegatecathode circuit of the SCR Q2 by a transformer T2.

When SCRs Q1 and Q2 are triggering alternately, the circuit 41 operates,as described in greater detail hereinafter to apply respective steppedvoltage waveforms to the electrode band 33 and to the screen 19, the twovoltages being switched out-of-phase with each other. Electrons emittedby gun 27 during the different time intervals corresponding to the twodifferent voltage levels of the waveform applied to screen 19 are thusaccelerated to different energy levels before reaching phosphor screen19. The energy levels are chosen in relation to the characteristics ofthe phosphors which make up screen 19 so that the lower energy electronsexcite only the red phosphor while the higher energy electrons exciteboth the red and cyan phosphors, thereby causing white light to beemitted.

The frequency of the rectangular wave is adjusted and synchronized byappropriately controlling the triggering of SCRs Q1 and Q2 so that thedifferent accelerating voltages are produced during periods whichcorrespond to the sequencing of the color video signal applied to gun27. The beam current is thus modulated to reproduce the various imagecomponents in their respective colors. In the example illustrated thisis assumed to be at a linesequential rate.

Registration between the different color image components is maintainedin the folowing manner: During the display of a white line, the screen19 is driven to the higher of its two potential levels and electronsemitted from gun 27 are accelerated to a relatively high energy level.These electrons thus produce white light when they strike the phosphorscreen 19 as explained previously. As the screen 19 is driven to itsmore positive voltage, the electrode band 33 is driven to its morenegative voltage level. Accordingly, electrons emitted from gun 27during this period are not greatly accelerated as they first leave thegun but rather attain only a relatively low velocity in the region ofthe yoke 29. These electrons are thus relatively highly subject todeflection by the yokes field and therefore follow a path having anearly high curvature as represented at A in FIGURE 1. As these electronsleave the vicinity of electrode 33, however, they are subjected to arelatively intense electric field and are thus accelerated to approachscreen 19 at a relatively steep angle, impinging at a point indicated atC.

When a red line is being displayed, the screen 19 is driven to the lowerof its two voltage levels. The total acceleration undergone by electronsemitted by gun 27 during this line period is thus relatively small andonly the red phosphor is energized. While the screen is at its lowervoltage level, the electrode band 33 is driven to the higher of its twovoltage levels and thus electrons emitted from gun 27 are rapidlyaccelerated as they first leave the gun, These electrons are thus notgreatly deflected in the region of the yoke and therefore follow a pathsubstantially as indicated at B in FIGURE 1. However, as screen 19 isthen at the lower of its two voltage levels, these electrons are notgreatly further accelerated before reaching the screen and thereforeapproach the screen substantially at the angle determined by theirearlier deflection, striking the screen substantially at the same pointC as the higher energy electrons following the path A.

As the particular configuration of kinescope 11 will affect thedistribution of the electric fields within its envelope, as will theboundaries of the electrode band 33 and the aluminum screen coating 21,it may be seen that the particular D.C. biasing voltages and theamplitudes of the steps between the two voltage levels applied to theseelements will vary from tube to tube in order to achieve bestregistration. Typically, the amplitude of the steps bet-ween the twovoltage levels will be different for the electrode 33 and the screen 19,the windings W1 and W2 being shown of different size in FIGURE 1 forthis reason.

The operation of circuit 41 may be understood by referring to theequivalent circuit diagram in FIGURE 2. 'In this diagram thetransforming effect of the turns ratio of transformer T1 is neglectedand the capacitive loads constituted by the electrode band 33 and thescreen 19 are represented by a single capacitance CL. The transformer T1is represented in FIGURE 2 by an inductance -LM which represents themutual inductance between the pri mary and secondary windings and a pairof inductances LLP and LLS which represent the leakage inductancesattributable to the primary and secondary windings respectively. At theswitching frequencies involved, the impedance of the inductance LM isrelatively large and, essentially, this component has no effect on theoscillatory or AC. mode of operation described hereinafter. Similarly,the capacitance C2 is relatively large so that, at the switchingfrequencies involved, its impedance is relatively low and it does notsubstantially effect the transfer of AC. energy.

FIGURE 3 represents the behavior of the voltage at a point X in thecircuit of FIGURE 2, which voltage may be taken as representative of thestepped component of the voltages applied to electrode 33 and screen 19.FIG- URE 4 similarly represents the current flowing into the capacitiveload CL.

When a positive supply voltage VS is first applied to the terminal 45,neither of the SCRs Q1 or Q2 conducts and the capacitances CL and C2remain uncharged. When SCR Q2 is triggered, the point X rises to thesupply voltage VS, as shown in FIGURE 3. Then, when SCR Q1 is triggered,the leakage reactances LLP and LLS and the load capacitance CL react inan oscillatory manner so that the voltage at point X behaves essentiallyas illustrated by the portion K-L of the curve of FIGURE 3. As may beseen, the curve between these points comprises 180 of a sine-wavefunction, the period of which is determined by the relative magnitudesof the leakage reactances LLP and LLS and the load capacitance CL. As isunderstood by those skilled in the art, the phase of the current flowingthrough capacitance CL is shifted 90 with respect to the applied voltageand thus behaves substantially as represented by section K' of the curveof FIGURE 4. At the end of the period K'L', the oscillatory or resonantcircuit comprising reactances LLP and LLS and capacitance CL tends todrive the current in the reverse direction. However, since Q1 canconduct in one direction only, it therefore ceases conduction andappears as an open circuit to the reactive circuit elements in seriestherewith. The voltage at point X therefore persists or remains at thelevel of point L until the SCR Q2 is triggered at the point of timeindicated at M.

At time M the voltage across SCR Q2 is equal to twice the supply voltagesince the point X is charged to a voltage which is equal and opposite tothe supply voltage with respect to ground. When SCR Q2 is thentriggered, the voltage of the point X behaves in the oscillatory mannerrepresented by the portion of the curve of FIG- URE 3 between the pointsM and N, this portion of the curve comprising a 180 of a sine-wavecentered about the supply voltage VS. The current flowing through loadCL during this interval behaves substantially as represented by theportion M of the curve of FIGURE 4. At the end of the interval M whenthe oscillatory circuit tends to reverse the flow of current, SCR Q2 isreverse biased and ceases conduction. The voltage at the point X thuspersists at the level which it had reached at the point N. The voltageacross SCR Q1 at this time is, as may be seen from FIGURE 3,substantially equal to three times the supply voltage VS. Thus when SCRQ1 is again triggered, the oscillatory action of the circuit causes thevoltage at the point X to swing to a negative voltage whose absolutemagnitude is substantially equal to three times the source voltage.

From the preceding explanation it can be seen that the leakagereactances LLP and LLS together with the capacitive load CL constitutean oscillatory system which can be pumped by alternate triggering of theSCRs Q1 and Q2 so that the energy stored in the oscillatory system isprogressively increased. However, since current can flow through theoscillatory system only by passing through one of the SCRs Q1 and Q2,the oscillatory or sinusoidal waveform is interrupted at various points,e.g., the intervals L-M and N-O, until the next SCR is triggered. Inactual practice these intervals may be made quite long relative to thetime required for switching the voltage so that an essentially steppedwaveform is obtained in which the voltage changes relatively abruptlyfrom one discrete level to another.

Assuming an entirely lossless system, the voltage at point X would buildup substantially according to the following progression:

However, the system necessarily involves some losses and thus thewaveform builds up only util the power consumed equals the powerdissipated by the losses and then the waveform reaches a stabilizedlevel. This build up and stabilization of the waveform is represented inFIGURE 5 As noted previously, the inductance LM and the capacitance C2have little effect on the AC. components of the waveform at theswitching frequencies involved. When the positive and negative of thewaveform are of substantially equal duration, the capacitor C2 takes onvery little D.C. charge. However, it is an advantage of the presentcircuit that it may be employed where the positive and negative portionsof the Wave are not of equal duration. Such operation may be highlydesirable in color display systems of the type illustrated in FIGURE 1when it is desired to display two lines of one color for every one lineof the other color. Such an operation may be expedient to obtain adesirable color balance or to cause the system to operate at a desiredmultiple of the verticle scanning rate so that so-called flicker or linecrawl, phenomena are avoided. When the SCRs Q1 and Q2 are triggered toproduce unequal durations of the positive and negative portions of thestepped voltage wave the capacitor C2 charges to a DC. voltage whichbalances the net DC. current flow through the SCRs Q1 and Q2 therebycausing a satisfactory waveform to be produced.

Voltage switching apparatus of this invention may also be modified, bythe addition of a voltage divider and a third SCR, to provide a thirdvoltage level step intermediate the extreme voltage steps. The threeSCRs are then triggered sequentially, the time of triggering of thethird SCR being in between the alternate triggering of the other two.This third level may, for example, be useful in threecomponcnt colordisplays. Accordingly, as used herein, the term stepped voltage shouldbe understood to include both a rectangular or square wave and also awave having three or more voltage steps at different levels.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

What is claimed is:

1. Voltage switching apparatus for applying a stepped voltage to acapacitive load, said apparatus comprising:

a transformer having a primary winding and a secondary winding, saidsecondary winding being adapted to be connected to said load;

a first switching circuit for connecting said primary winding across avoltage source, said first switching circuit including an SCR and acapacitor in series with each other and with said primary winding;

a second switching circuit including a second SCR which is connectedacross said primary winding and said capacitor from a first junctionbetween said SCR of said first switching circuit and said primarywinding to a second junction on the side of said capacitor and primarywinding opposite said SCR of said first switching circuit; and

means for applying signals to the gate terminals of said SCRs fortriggering said SCRs into conduction alternately, said transformerhaving leakage reactance which, upon triggering of either of said SCRs,causes said capacitive load to be charged to a respective voltage whichthen reverse-biases that SCR thereby cutting off conduction therein,whereby said load is switched repetitively between at least twodifferent voltage levels each of which persists until the next SCR istriggered.

2. Voltage switching apparatus as set forth in claim 1 including acapacitor for coupling triggering signals to the SCR in said firstswitching circuit.

3. Voltage switching apparatus as set forth in claim 1 including atransformer for coupling triggering signals to said second SCR.

4. Voltage switching apparatus as set forth in claim 1 wherein saidtransformer comprises a pair of secondary windings for providingout-of-phase stepped voltages to the conductors of said capacitive load.

5. A color display system comprising:

a screen including phosphors which emit light of different colors whenstruck by electrons of different energies said screen forming acapacitive electrical load;

an electron gun for emitting a beam of electrons toward said screen;

field generating means for deflecting said beam of electrons in ascanning raster;

means for applying a DC. bias voltage to said screen to accelerateelectrons emitted from said gun toward said screen;

a transformer having a primary winding and a secondary winding, saidsecondary winding being adapted to be connected to said screen forapplying a time-varying voltage thereto;

a first switching circuit for connecting said primary winding across avoltage source, said first switching circuit including an SCR and acapacitor in series with each other and with said primary winding;

a second switching circuit including a second SCR which is connectedacross said primary winding and said capacitor from a first junctionbetween said SCR of said first switching circuit and said primarywinding to a second junction on the side of said capacitor and primarywinding opposite said SCR of said first switching circuit; and

means for applying signals to the gate terminals of said SCRs fortriggering said SCRs into conduction alternately, said transformerhaving leakage reactance which, upon triggering of either of said SCRs,causes said capacitive load to be charged to a respective voltage whichthen reverse-biases the triggered SCR thereby cutting off conductiontherein, whereby said screen is switched repetitively between at leasttwo different voltage levels each of which persists until the next SCRis triggered and whereby said electrons are accelerated to at least twodifferent energies for energizing said screen to produce multiple-colorimages.

6. A color display system comprising:

a screen including phosphors which emit light of differcut colors whenstruck by electrons of different energies;

an electron gun for emitting a beam of electrons toward said screen;

field generating means for deflecting said beam of electrons in ascanning raster;

a generally annular electrode concentric with said gun and axiallydisplaced therefrom toward said screen and into the region of the fieldproduced by said field generating means, said screen and said electrodeforming capacitive electrical loads;

means for applying DC. bias voltages to said screen and said electrodeto accelerate electrons emitted from said gun toward said screen;

a transformer having a single primary winding and secondary windingsadapted to be connected to said screen and said electrode for applyingout-of-phase time-varying voltages thereto;

a first switching circuit for connecting said primary winding across avoltage source, said first switching circuit including an SCR and acapacitor in series with each other and with said primary winding;

a second switching circuit including a second SCR which is connectedacross said primary winding and said capacitor from a first junctionbetween said SCR of said first switching circuit and said primarywinding to a second junction on the side of said capacitor and primarywinding opposite said SCR of said first switching circuit; and

means for applying signals to the gate terminals of said SCRs fortriggering said SCRs into conduction alternately, said transformerhaving leakage reactance which, upon triggering of either of said SCRs,causes said capacitive loads to be charged to respective voltages whichthen reverse bias the triggered SCR thereby cutting off conductiontherein, whereby said screen and said electrode are switchedrepetitively between .at least two different respective voltage levelseach of which persists until the next SCR is triggered and whereby saidelectrons are accelerated to at least two different energies forenergizing said screen to produce multiple-color images having colorimage components which are in substantiial registration.

7. A color display system as set forth in claim 6 wherein said screenand said electron gun are supported within a kinescope envelope andwherein said electrode comprises a conductive layer on the inner surfaceof said envelope.

8. A color display system as set forth in claim 6 wherein saidtransformer comprises separate secondary windings for said screen andsaid electrode and wherein said DC. bias voltages are applied to thescreen and said electrode through the respective windings.

References Cited UNITED STATES PATENTS 2,741,526 4/1956 Lalferty 3l5143,005,927 10/ 1961 Godfrey 313-76 X 3,109,956 11/1963 Stratton 3l5143,114,795 12/1963 Moles 3l5l4 X 3,225,238 12/1965 Feldman 313923,290,435 12/1966 Shimada 3 l5--l4 5 RICHARD A. FARLEY, PrimaryExaminer.

M. F. HUBLER, Assistant Examiner.

US. Cl. X.R.

Notice of Adverse Decisions in Interferences In Interference No. 97,206involvin Patent No. 3,424,939, W. H. Clingman, Jr., VOLTAGE SWITCHING A%PARATUS FOR COLOR KINE- SCOPES, final judgment adverse to the patenteewas rendered J an. 19, 1973, as to claims 39 and 4:0.

[Ofiicz'al Gazette J uly 10,1973]

